This invention relates to switch circuits for battery-powered equipment, for example mobile telephones or portable computers. The invention also relates to 4-terminal bi-directional semiconductor switches suitable for use in such switch and to battery-powered equipment comprising such switches and switch circuits.
United States patent specification U.S. Pat. No. 5,682,050 discloses several switch circuits for a battery-powered equipment that comprise a 4-terminal bi-directional switch and a protection diode. The switch has a main current path between first and second main terminals that serve for coupling the switch between a battery and a power line of the equipment. The switch has a control-gate terminal that serves for applying a control signal to the switch to form a conduction channel in a body region of the switch, the conduction channel forming part of the main current path for turning the switch on and off. The switch has a back-gate terminal in a bias path that serves for applying a bias potential to the body region. The whole contents of U.S. Pat. No. 5,682,050 are hereby incorporated herein as reference material.
Included in U.S. Pat. No. 5,682,050 is a disclosure of circuitry to protect the switch and battery in the event of a reverse voltage polarity across the first and second main terminals of the switch. Such an event can occur if, for example, the switch circuit is connected to a battery charger with reversed connections, i.e. with the wrong polarity. The body region of the switch acts as a base region of a parasitic bipolar transistor coupled in parallel with the switch.
FIGS. 12B and 12C of U.S. Pat. No. 5,682,050 show protection circuitry having a protection diode (a Schottky diode) that has a diode path coupled to a terminal of the switch, namely to a particular one of the first or second main terminals. This particular one main terminal is the one that is not connected to the battery but is connected to the battery charger.
In the FIG. 12B circuit, the current through the diode is limited by a resistor RB that connects the diode to the back-gate terminal at ground potential. In the FIG. 12C circuit, the diode is connected directly to ground, independent of the back-gate terminal. In each case, the intention is to prevent turn on of the parasitic bipolar transistor. U.S. Pat. No. 5,682,050 rejects these simple diode circuits as being unable to provide adequate protection for the switch and the battery. Instead, U.S. Pat. No. 5,682,050 proposes a more complex protection circuit (FIG. 13) with three MOSTs. In all the protection circuits shown in U.S. Pat. No. 5,682,050 (FIGS. 12B to 12D, 13A to 13E, and 14A and 14B), the body region of the switch is connected to the back-gate terminal at ground potential via a/the resistor RB.
It is an aim of the present invention to provide switch circuits and 4-terminal switches with a protection diode arrangement that can be simple and yet can provide adequate protection in the event of a reverse voltage polarity across the first and second main terminals of the switch.
Switch circuits and/or switches in accordance with the present invention include a protection diode having a diode path in series with the back-gate terminal of the switch so as to provide a rectifying barrier in its bias path. This rectifying barrier blocks current flow between the body region of the switch and its back-gate terminal in the event of a reverse voltage polarity across the first and second main terminals of the switch. Thus, in the situation where the body region of the switch would act as a base region of a parasitic bipolar transistor, then the rectifying barrier of the protection diode in accordance with the invention blocks base current to the body region (parasitic base) from the back-gate terminal.
A Schottky barrier or a p-n junction may form the protection diode. The switch and the protection diode may be separate components each comprising a semiconductor body with its own terminals. Alternatively, the protection diode may be integrated with a semiconductor device body of the switch or of, for example, a control IC (integrated circuit).
Thus, for example, the present invention provides embodiments of novel switch circuits having the features set out in claims 1 to 3, and there are also provided embodiments of novel 4-terminal bi-directional semiconductor switches (with integrated protection diodes) having, for example, the features set out in claims 4 to 9.